1. Field of the Invention
This invention relates to an antenna in use for wireless communication, and in particular, to a dielectric antenna having a so-called meander line configuration formed on a dielectric substrate for use in high frequency wireless communication.
2. Related Art
An antenna is an indispensable constituent element in wireless communication but has the disadvantage of consuming or occupying substantial space, relatively speaking. To reduce the size of the antenna, known antenna configurations use a dielectric material and form an antenna line on or within the dielectric material. An example of antennas according to this prior art is described in Japanese Patent Laid-Open No. 13126/1998. To suppress or reduce exothermy (heat evolution) resulting from a power loss from a radiation electrode and to provide an antenna having reduced wavelength fluctuation, the antenna has a construction as shown in FIG. 13. Referring to FIG. 13, the antenna 2100 includes a spiral radiation electrode 2102 formed on an outer surface of a dielectric insulator 2101, a further electrode 2103 electrically connected to the radiation electrode 2102 and having a line width greater than that of the radiation electrode 2102, and a feed terminal 2104 electrically connected to the electrode 2103 for supplying high frequency signals to the radiation electrode 2102 and to the further electrode 2103. According to this construction, the electrode 2103 releases the heat resulting from the power loss of the radiation electrode 2102, and the temperature rise of the radiation electrode 2102 and the insulator 2101 can be suppressed or reduced. As a consequence, fluctuation in the antenna wavelength can be reduced.
When the size of the antenna is reduced to provide miniaturization, i.e., to satisfy a space requirement, the width of the antenna line, i.e., the conductor forming the antenna, becomes quite small. When the antenna line is of a linear shape, the physical length of the antenna must increase. Therefore, to save space, the antenna line is formed in an undulating shape, such as is illustrated, for example, in Japanese Patent No. 3,114,582 and Japanese Patent Laid-Open Nos. 55618/1997 and 139621/1997. In such antennas, the line width of the antenna is likely to be further reduced in order to provide a decrease in antenna length by use of the undulating shape.
Generally speaking, the greater the number of components, the greater the size of the antenna because of the need for impedance matching. Moreover, impedance mismatching with the line on the component packaging substrate is more likely to occur, thereby resulting in deterioration of the radio wave radiation characteristics. In other words, it is more difficult to efficiently transmit the high frequency signals supplied from a feed terminal to the antenna line. The length of the antenna line is generally adjusted to control such impedance mismatching. However, given ever more demanding space requirements, i.e., due to the need for miniaturization, the antenna line length cannot always be arbitrarily changed. Further, while it is known to insert a matching circuit between the line on the component packaging substrate side and the antenna line, the addition of such a matching circuit tends to increase production costs and to consume excessive space, which is, of course, contrary to the need for miniaturization.
There are various factors causing impedance mismatching between the antenna line and the feed terminal portion. For instance, when, due to design limitations, the antenna line width is different from the width of a feed strip line for signal transmission, and particularly when the width of the antenna line is smaller than that of the feed strip due to miniaturization of the antenna, a problem with impedance mismatching is most likely to occur.